In recent years, for the purposes of ensuring the safety of occupants in crash and improving a fuel efficiency because of car body weight reduction, application of a high strength steel sheet having a TS of 780 MPa or more and a small sheet thickness to an automobile structural member has been promoted intensively. Particularly in recent years, application of an extremely high strength steel sheet having a TS of a 980 MPa level or a 1,180 MPa level has been discussed.
However, in general, an increase in strength of a steel sheet causes deterioration of the elongation, the stretch frangeability, the bendability, and the like of the steel sheet and leads to deterioration of the formability. Therefore, a galvanized steel sheet having the high strength and excellent formability in combination and, furthermore, exhibiting excellent corrosion resistance has been desired under the present circumstances.
In response to such demands, for example, JP 9-13147 proposes a high strength galvannealed steel sheet having a TS of 800 MPa or more and exhibiting excellent formability and excellent adhesion of the coating, wherein a galvannealed layer is provided on a surface layer of a steel sheet containing C: 0.04% to 0.1%, Si: 0.4% to 2.0%, Mn: 1.5% to 3.0%, B: 0.0005% to 0.005%, P≦0.1%, 4N<Ti≦0.05%, Nb≦0.1%, and the remainder composed of Fe and incidental impurities, on a percent by mass basis, the Fe % in the galvannealed layer is 5% to 25%, and the microstructure of the steel sheet is a mixed microstructure of a ferrite phase and a martensite phase.
JP 11-279691 discloses a high strength galvannealed steel sheet exhibiting good formability, wherein C: 0.05% to 0.15%, Si: 0.3% to 1.5%, Mn: 1.5% to 2.8%, P: 0.03% or less, S: 0.02% or less, Al: 0.005% to 0.5%, N: 0.0060% or less, and the remainder composed of Fe and incidental impurities are contained, on a percent by mass basis, (Mn %)/(C %)≧15 and (Si %)/(C %)≧4 are satisfied, and 3% to 20% of martensite phase and retained austenite phase on a volume fraction basis are contained in a ferrite phase.
JP 2002-69574 discloses a high strength coated steel sheet with low yield ratio, the steel sheet exhibiting excellent stretch frangeability, wherein C: 0.04% to 0.14%, Si: 0.4% to 2.2%, Mn: 1.2% to 2.4%, P: 0.02% or less, S: 0.01% or less, Al: 0.002% to 0.5%, Ti: 0.005% to 0.1%, N: 0.006% or less, and the remainder composed of Fe and incidental impurities, on a percent by mass basis, are contained, (Ti %)/(S %)≧5 is satisfied, the volume fraction of a martensite phase and a retained austenite phase is 6% or more in total, and α≦50,000×{(Ti %)/48+(Nb %)/93+(Mo %)/96+(V %)/51} is satisfied, where the volume fraction of a hard phase microstructure of the martensite phase, the retained austenite phase, and a bainite phase is assumed to be α%.
JP 2003-55751 discloses a high strength galvanized steel sheet excellent in adhesion of the coating and elongation in formation, the galvanized steel sheet including a coating layer containing Al: 0.001% to 0.5%, Mn: 0.001% to 2%, and the remainder composed of Zn and incidental impurities, on a percent by mass basis, on a surface of a steel sheet containing C: 0.001% to 0.3%, Si: 0.01% to 2.5%, Mn: 0.01% to 3%, Al: 0.001% to 4%, and the remainder composed of Fe and incidental impurities, on a percent by mass basis, wherein a Si content of steel: X percent by mass, a Mn content of steel: Y percent by mass, an Al content of steel: Z percent by mass, an Al content of coating layer: A percent by mass, and a Mn content of coating layer: B percent by mass satisfy 0≦3−(X+Y/10+Z/3)−12.5×(A−B), and the microstructure of the steel sheet includes 70% to 97%, on a volume fraction basis, of primary ferrite phase having an average grain diameter of 20 μm or less and 3% to 30%, on a volume fraction basis, of austenite phase and/or martensite phase serving as a secondary phase having an average grain diameter of 10 μm or less.
However, regarding high strength galvanized steel sheets described in JP 9-13147, JP 11-279691, JP 2002-69574 and JP 2003-55751, excellent stretch frangeability and bendability are not always exhibited.
In consideration of the above-described circumstances, it could be helpful to provide a high strength galvanized steel sheet having a TS of 780 MPa or more and exhibiting excellent stretch frangeability and bendability and a method for manufacturing the same.